The present invention relates to a diffuser that surrounds an impeller of a centrifugal compressor, and in particular to a diffuser arrangement including a plurality of tangential passages formed in an annular member surrounding the impeller circumferentially at a regular interval for converting the velocity of the fluid exiting the impeller into the pressure thereof.
A centrifugal compressor is provided with a diffuser disposed around the impeller for decelerating the fluid flow from the impeller and converting the dynamic pressure of the fluid flow into a static pressure. A diffuser normally defines a plurality of passages which extend across a circumferential circle defined around the impeller. Such passages are most commonly defined by stator vanes that extend between a pair of walls. Another form of diffusers is called as xe2x80x9ca pipe diffuserxe2x80x9d which consists of a plurality of passages each having a circular cross section. Such a diffuser may be formed by drilling passages in an annular member, and a diffuser of this type is disclosed in U.S. Pat. No. 5,145,317.
In such a pipe diffuser, as shown in FIGS. 8 and 9, an arcuate leading edge 26 is formed between each adjacent pair of passages 23 which intersect each other at an acute angle at point Axe2x80x2 (as seen on the projected plane in parallel with the paper of FIG. 8). According to such a conventional diffuser, the passages 23 are formed in such a manner that the longitudinal center line 24 of each passage 23 tangentially touches the outer peripheral circle 28 of the impeller 22 at point Bxe2x80x2. Therefore, the longitudinal center line 24 is at a right angle to the radial line OBxe2x80x2 of the impeller 22. However, the front parts 26a and 26b of the leading edge 26 which coincide with point Axe2x80x2 in FIG. 8 are spaced away from the outer peripheral circle 28 of the impeller 22 by a certain distance, and this distance increases as the number of passages decreases. Therefore, in a diffuser arrangement using a small number of passages, this distance creates an incidence mismatch which could cause a blockage of a throat section of each passage 23, and impair the efficiency in recovering static pressure.
Also, when drilling the passages, protrusions 30 tend to remain between the front ends or side ends 26a and 26b of leading edge 26 and the inner peripheral part of the diffuser, and the need for removing such protrusions 30 increases the manufacturing cost.
In view of such problems of the prior art, a primary object of the present invention is to provide a diffuser arrangement for centrifugal compressors which can substantially reduce the incidence mismatch, and can thereby improve the efficiency in recovering the static pressure.
A second object of the present invention is to provide a diffuser arrangement for centrifugal compressors which provides a high efficiency even when the number of passages is relatively small.
A third object of the present invention is to provide a diffuser arrangement for centrifugal compressors which allows the machining work to be simplified.
According to the present invention, such objects can be accomplished by providing a diffuser arrangement for a centrifugal compressor, comprising a member surrounding an impeller of the compressor and defining a plurality of passages extending substantially tangentially around the impeller at a regular interval circumferentially, wherein: a longitudinal center line of each of the passages is tangential to a reference tangent circle defined concentrically with respect to the impeller and having a smaller diameter than an outer diameter of the impeller so that a leading edge defined at an intersection between each pair of adjacent passages is brought close to an outer periphery of the impeller.
Because the leading edge, in particular the side portions thereof, is brought close to the outer peripheral part of the impeller, the incidence mismatch can be minimized so that a blockage in the throat section of each passage can be avoided, and the static pressure can be recovered at a high efficiency. Also, when drilling the passages, protrusions are not formed in the inner peripheral part of the diffuser. Elimination of the need to remove such protrusions contributes to the reduction in the manufacturing cost.
Preferably, the radius of the reference tangent circle or Rtan is given by Rtan=RIMPxc3x97cos (xcfx80/N)+xcex4 where RIMP is the radius of the impeller, N is the number of passages, and xcex4 is a small gap for providing a clearance between the impeller and diffuser.
Preferably, the number of passages is between 10 and 22. If the number is less than 10, the wavy shape in the inlet part of the diffuser becomes so pronounced that a substantial mismatch is created between the impeller outlet and the diffuser inlet. If the number is greater than 22, the radius of the reference tangent circle would differ from that of the outer periphery of the impeller by no more than 1%, and the resulting gain would not be appreciable.
The diffuser of the present invention can be fabricated easily but still can demonstrate a high efficiency. According to such an embodiment, the passages may consist of holes drilled in an annular member disposed concentrically with respect to the impeller. Each of the passages may be slightly flared outwardly at least in a radially outer part thereof. Alternatively, each of the passages may be provided with a rectangular or other polygonal cross section or elliptic cross section without departing from the spirit of the present invention. In such a case, the diffuser may be also formed by casting, forging, machining or other metal working processes.